Neutron detection and radiography

ABSTRACT

An improved method of recording neutron images which comprises imagewise irradiating with neutrons an intesifying screen containing a gadolinium compound that fluoresces when struck by x-rays and subjecting the fluorescent light pattern resulting from the impact of the neutrons on the screen onto a photographic material.

United States Patent Bollen et al.

[ June 24, 1975 I54] NEUTRON DETECTION AND 3,140,397 7/l964 Henry 250472 RADIOGRAPHY 3.617.285 l ll|97l Staudenmaycr 250/483 X 3.6l7,743ll/l97l Rabatin ct al. 250/483 X [75] In entors: Romain Henri B llen, HRobert 3,6l7,747 |971 Wilkinson et al 250 391 x figz' r i" Esch'Meche'en' bmh OTHER PUBLICATIONS Radiation Shielding, by Price et al.,Pergamon Press [73] Asslgnee: Agta -Gevaert, N.V., Mortsel. 1957 p, 333,334, 335.

Belgium '22] Filed: N0 3. 1972 Primary Examiner-Archie R. BorchcltAttorney, Agent, or FirmAlfred W. Breiner [2|] Appl. N0.: 303,387

[57] ABSTRACT [52] US. Cl 250/39]; 250/483 An improved method ofrecording neutron images [5 l] Int. Cl. G0lt 3/00 which comprisesimagewise irradiating with neutrons [58] Field of Search..... 250/312VT. 483, 486. 390, an intesifying screen containing a gadolinium com-250/39], 392 pound that fluoresces when struck by x-rays and subjectingthe fluorescent light pattern resulting from the [56] References Citedimpact of the neutrons on the screen onto a photo- UNITED STATES PATENTSgraphic material- 3.076 895 2/[963 Baldwin 250/392 29 Claims, 1 DrawingFigure I 4" A L b e 4 A *3 7 b 4 I 0 o o o o g o o o a 2 6 060 no 1NEUTRON DETECTION AND RADIOGRAPHY This invention relates to thedetection of neutrons and the recording of neutron images and to the useof particular substances, materials and devices in said detection andrecording The interest in neutron recording and detection in cludesamong others neutron dosimetry, neutron diffraction, neutron radiographyand neutron image conversion in a visible image e.g., in an imageintensifier tube. Neutron radiography is a well known valuablecomplement to X-ray and gamma ray radiography.

As is known e.g., from H. Kallmann, Research I, (1948) Nr. 6, pages254-260, neutrons do not substantially affect a photosensitive silverhalide directly. Therefore in common neutron radiography a combinationof silver halide film with a neutron sensitive intensifying screen isapplied.

In general, neutron radiatiomsensitive screens can be divided into threeclasses according to the function they have to perform.

The first class includes screens which are called here "neutronconversion screens." Such screens have the property to produceultraviolet light or visible light photons in a fluorescent screencontacting a photocathode that emits photoelectrons under the influenceof the fluorescent light. Such screens may find application in imageintensifier or image conversion tubes similar to those known from X-rayradiography (ref. 0.]. Van der Plaats, Medical X-ray Technique (1959)Phillips Technical Library, pages l03-l08). in such tubes thephoto-electrons emitted by the photocathode are acclerated electricallyin vacuum and an electric or magnetic focusing field is used to focusthe electron image on another fluorescent screen which is sensitive forfast moving electrons. The photo-electrons are thus transformed intovisible light in a way similar to that known from television picturetubes.

The second class includes screens which are called here fluorescopicscreens." Such screens have the function of producing a directlyviewable image in correspondence with the neutron image. The screen isviewed e.g., by means of optical means or photographed for the purposesof cineradiography or for producing television pictures. Thefluorescopic image may be intensified with an electronic imageintensifying tube before being viewed or phtotgraphed.

The third class includes screens which are called here intensifyingscreens." Such screens have the function to transform the neutronradiation into electromagnetic radiation or high energy particleradiation e.g., B or a-rays, for which a photographic material directlycontacting the screen is sensitive.

Normally the intensifying screens are used in combination with silverhalide emulsion film sheets that together and in immediate contact withthe screen(s) are mounted in a light-tight cassette.

There are two film-exposure techniques. in one, the "direct-exposuremethod, the photographic film and the screen in which the neutron energyis converted or transformed in energy for which the photographic film issensitive are exposed together to the neutron beam to be detected orrecorded, whereas in the other method, the "transfer method, only ascreen is exposed to the neutron beam; by the neutron bombardment thescreen obtains a radioactive pattern corresponding with the neutron beampattern. This radioactive film is placed next to the photographic film.The film is then exposed by the decay radiation from the screen. (ref.Nucleonics, 20 (September i962) No. 9, page 77).

There are three basic types of intensifying screens used in thermalneutron radiography, viz. granular, glass and metal foil screens. M. R.Hawkesworth et al. in the Journal of Scientif. instruments 3 (November1970) Nr. ll, pages 85 l-854, describes as granular intensifying screena screen containing a l:2:l (by weight) mixture of lithium-6 fluoride,zinc sulphide and perspex having a thickness of 0.65 mm, as metal screena 25 um foil of gadolinium and as glass screen a sheet oflithium-6 oxideloaded, cerium activated, scintillator glass l.3 mm in thickness.

The metal foil screens have to be divided in two types viz. metalscreens for direct exposure and metal screens for transfer exposure.

The most useful metal screen for direct exposure is the gadolinium metalscreen, which has a very high thermal neutron absorption coupled withthe immediate emission ofB-particles of low energy keV). The screen hastherefore extremely-high resolving power. However, the 70 keV electronscarry only 1/68 of the energy of the Li(n,a )T taking place in the abovementioned granular lithium-6 fluoride-zinc sulphide screen. The basicreaction in such a screen is Li n ,T a 4.79 MeV. Taking into account therather low activity of the gadolinium metal screens the fastest silverhalide films have to be used in conjunction therewith. The gadoliniummetal screens are insufficiently selective to detect neutrons in thepresence of 'y-rays. Therefore the metal foil screens in order to giveperfect discrimination against y-rays are used in the transfertechnique. Other ways to exclude 'y-radiation outside of the nuclearreactor are described in The Journal of Photographic Science, Vol. l9,i971, page 108.

Most metals, unlike gadolinium, emit B-particles some time after neutronabsorption. If such a foil is exposed behind an object in a neutron beamthe neutron radiographic image will be stored in the foil, which can beremoved from the beam after exposure and pressed against a suitablephotographic film. The radiograph will then be produced as the foilactivity decays. The metals most often used for this work are dysprosiumand indium. The transfer technique takes more time than the directexposure technique.

it is an object of the present invention to provide a neutron detectionand recording technique of improved speed.

It is another object of the present invention to use in neutrondetection and neutron radiography a combination of photographicmaterials and fluorescent substances that is particularly useful forthat type of detection and recording.

Other objects and advantages of the present invention will becomeapparent from the further description and example.

It has now been found that gadolinium compounds that have the propertyof fluoreseing when struck with X-rays have a very effective neutronstopping power and have the property of very efiectively transformingcaptured neutron radiation into visible or near visible radiation forwhich silver halide is inherently sensitive or can be made sensitive byspectral sensitization.

Said gadolinium compounds as can be learned from comparative tests havea much larger "relative intensiflcation factor" than pure gadoliniummetal intensifying screens.

By "relative intensification factor" is to be understood here, a factormeasured at a pre-elected density D, indicating the dosis (neutrons percm2) required to produce this density when a silver halide film isexposed to neutrons with a standard screen divided by the dosis requiredto produce the same density with the screen to be tested. Exposureconditions, film and developing conditions are kept constant for thecomparative tests.

In accordance with the present invention it has been discovered thatgadolinium compounds of the oxide, oxyhalide, and oxysulphide type areparticularly valuble substances for the detection and recording ofneutron beams. Preferred substances for the purpose of the inventioncontain gadolinium as "host" metal in a phosphor in which (an) otherrare earth metal(s) is (are) present as fluorescence activating metal."

Visible light emitting neutron detecting screens applied according tothe present invention are preferably gadolinium oxysulphide or oxyhalidefluorescing substances activated with at least one other selected rareearth metal e.g., terbium, dysprosium, erbium, europium, holmium,neodymium, praseodymium, samarium, thullium or ytterbium.

Particularly suited phosphors for use in neutron radiography correspondto the following general formula:

wherein:

M is at least one of the metals terbium, dysprosium,

erbium, europium, holmium, neodymium, praseodymium, samarium, thulium orytterbium,

X is sulphur or halogen e.g., chlorine, bromine or fluorine,

n is 0.0002 to 0.2, and

w is 1 when X is halogen or is 2 when X is sulphur.

The preparation of Rare Earth Oxyhalides and Oxide Luminescent Materialshas been described, e.g., by l. G. Rabatin in US. Pat. No. 3,607,770 andin the French Pat. No. 2,021,398.

Gadolinium oxyhalides activated with dysprosium are more particularlydescribed in the United States Patent Application Ser. No. 769,922,gadolinium oxyhalides activated with terbium are described in US. Pat.No. 3,617,743, gadolinium oxybromide activated with erbium has beendescribed in the US. Pat. No. 3,546,128. Fluoro-substituted europiumactivated gadolinium oxides have been described in the US. Pat. No.3,415,757.

Gadolinium oxysulfide activated with other rare earth elements has beendescribed in the US. Pat. No. 3,418,246 and by S. P. Wang et a1. IEEETransactions on Nuclear Science Vol. NS-17 (February 1970) p. 49-56; andby R. A. Buchanan IEEE Transactions on Nuclear Science, February 1972,pages 81-83.

Fluorescent gadolinium compounds that may be used according to thepresent invention are mentioned in the following documents the UnitedKingdom Pat. Nos. 1,018,005 1,022,930 1,284,296 1,131,956 1,110,2901,122,923 1,128,512 1,254,271 1,257,322 1,266,407 1,248,299 1,249,5441,279,450 1,247,602 1,263,038 and 1,269,329. in the US. Pat. Nos.3,415,757 3,434,863 3,301,791 3,282,856 3,502,590 3.562,]74 3,563,9093,484,381 3,418,246 3,418,247

3,634,282 3,546,128 3,574,131 3,574,129 and 3,661,791, in the CanadianPat. Nos. 799,899 and 816,628, in the published German Pat. Nos.1,284,296 1,222,610 1,592,884 1,915,360 1,935,103 2,051,262 2,051,2402,108,676 and 2,201,271, in the published Dutch Pat. No. 66/3921 and theFrench Pat. Nos. 1,468,075 1,501,441 1,504,341 1,550,113 1,580,5441,542,140 2,021,398 and 2,039,921.

Gadolinium compounds that emit in the ultra-violet and/or visiblespectrum range are very useful for neutron radiography witih silverhalide recording materials.

A high speed neutron sensitive intensifying screen contains e.g.,terbiumactivated gadolinium oxysulphide that has emission peaks at 490and 540 nm and falls within the scope of the above general formula.

Although in the fluorescent screens for use in neutron radiographyaccording to the present invention preferably only fluorescent compoundsare used that contain gadolinium, we do not exclude screens that containthe fluorescent gadolinium compound(s) in admixture with other phosphorsubstances that fluoresce when struck by X-rays and/or B-rays. Thus,e.g., a suitable fluorescent screen contains a mixture of 25:75 byweight of:

A. yttrium oxysulphide activated with from 0.1 to 10 by weight ofterbium or activated with terbium and dysprosium, and

B. gadolinium oxysulphide activated with terbium or dysprosium isparticularly useful for its high visible light emission through neutronexposure.

By spectral sensitization a silver halide recording material can be madeoptimally sensitive for the visible light e.g., green light emitted bythe fluorescent gadolinium compound.

By using a plurality of fluorescent gadolinium compounds in a pluralityof different screens or by using a fluorscent screen containing amixture of different gadolinium fluorescent compounds of the abovegeneral formula a fluorescence over the whole visible spectrum can beobtained, so that such combination is particularly useful for recordingwith silver halide recording elements that have been made spectrallysensitive for light of the whole visible spectrum.

The selected fluorescent gadolinium containing compound(s) is (are) inthe form ofa layer applied to a support or applied as a self-supportinglayer or sheet. Particularly suited fluorescent layers or sheets have athickness of preferably 10 to 300 microns and contain the fluorescentcompound(s) or phosphors e.g., dispersed in a binder or in the form of avapour deposited or sintered particle layer either or not in thepresence of a glassy material.

The binder if any is used, is e.g., an organic high molecular weightpolymer. Preferred binding agents are, e.g., cellulose nitrate, ethylcellulose, cellulose acetate, polyvinyl acetate, polystyrene,polyvinylbutyral, polymethylmethacrylate and the like.

The proportion of high molecular weight polymer binder to fluorescentmaterial is in general within the range of 5-15% by weight. A preferredgrain size of the fluorescent gadolinium compounds is in the range ofabout 1 to 20 microns.

The surface of the fluorescent material layer may be protected againstmoisture and mechanical damage by a coating of an organic high molecularweight polymer applied to a thickness of 0.001 to 0.05 mm. Suchprotecting coating is, e.g., a thin film of cellulose nitrate, celluloseacetate, polymethyl methacrylate and the like.

Besides the fluorescent light impinging normally to the silver halidecontaining layer there is always an amount of diffuse radiation in thefluorescent screen giving rise to image unsharpness. The image sharpnessis improved considerably by incorporating a fluorescent light-absorbingdye called here screening dye" into the fluorescent screen materiale.g., in the fluorescent layer or into an adjacent layer thereto, e.g.,a covering layer or subjacent antihalation layer. As the diffusedoblique radiation covers a larger path in the screen material it isattenuated by the screening dye to a greater extent than the radiationimpinging normally. The term screening dye includes here dyestuffs i.e.,coloured substances in molecularly divided form as well as pigments.

Diffuse radiation reflecting from the support of the fluorescent s'creenmaterial is mainly attenuated in an antihalation layer containing thescreening dyes subjacent to the fluorescent layer.

The use of screening dyes in a covering layer to the fluorescent layermainly reduces the strength of the obliquely emitted light originatingfrom the fluorescent layer.

An appropriate screening dye for use in the fluorescent screens emittingin the green part (500-600 nm) of the visible spectrum is, e.g.,Neozapon Fire Red (C.l. Solvent Red 119), an azochromium rhodaminecomplex. Other suitable screening dyes are C.l. Solvent Red 8, 25, 30,31, 32, 35, 7|, 98, 99, 100, 102, 109, 110,118,124 and 130.

The screening dye has not to be removed from the fluorescent screenmaterial and therefor may be any dye or pigment absorbing in theemission spectrum of the fluorescent substance(s). Thus a blacksubstance such as carbon black incorporated in the antihalation layer ofthe screen material yields quite satisfactory results.

The screening dye(s) is (are) preferably used in the antihalation layerin an amount of at least 0.5 mg per sq.m. Their amount in theanti-halation layer is not limited.

Very good results are obtained with the screening dye(s) in theantihalation layer and in the layer containing the fluorescentsubstances. In that case the fluorescent layer contains e.g., thescreening dye or dyes in an amount of 5 mg per sq.m. The amount ofscreening dye(s) in the fluorescent layer and/or covering layer may beadapted to the results of image sharpness and intensity of emittedradiation aimed at.

The present invention includes the use of fluorescent gadoliniumcompound-containing screens in neutronradiography in conjunction with another neutron absorbing screen e.g., of the metallic, glassy or granulartype. Thus, e.g., the combination with gadolinium, dysprosium, or indiummetal screens, the latter two screens being used e.g., in the mentionedtransfer technique." The invention includes likewise the use offluorescent gadolinium compound-containing screens in neutronradiography in conjunction with metal screens that have a relativelyhigh neutron absorption e.g., an absorption at least as high as lithiumin order to remove a certain amount of diffuse neutron rays to improvethereby image-sharpness. An analogous technique in which animage-sharpness improving metal screen is used in conjunction with afluorescent screen but for use in X-ray radiography has been describedin the United Kingdom Pat. No. 6l,050/71 filed Dec. 31, 1971 byAgfa-Gevaert N.V.

Metal screens with a high neutron absorption power may be made of themetals cadmium, gadolinium, europium and samarium or alloys thereof.

Preferred neutron radiographic combinations for use according to thepresent invention employ in addition to the fluorescent screen aphotosensitive element comprising a suitable support bearingphotosensitive silver halide. Said silver halide may be present in alayer or coating such as a single coating or a duplitized or dualcoating, i.e., in a material having a silver halide emulsion layer oneach side of a support. Suitable supports are those having theproperties to permit their ready passage through a rapid automaticprocessor. The support should therefore be reasonably flexible andpreferably transparent but able to maintain the dimensional stabilityand integrity of the various coatings thereon. Typical film supports arecellulose nitrate, cellulose ester, polyvinyl acetal, polystyrene,polyethylene terephthalate, and the like. Supports such as cards orpaper that are coated with a-olefln polymers, particularly polymers ofa-oleflns containing two or more carbon atoms, as exemplified bypolyethylene, polypropylene, ethylene-butene copolymers and the like,give good results.

In order to improve image sharpness dyes are used in the silver halideemulsion recording material which dyes are called hereinafter filteringdyes." They are preferably incorporated in a hydrophilic colloid layere.g., between the silver halide emulsion layers and/or in the emulsionlayers themselves. They may, however, likewise be incorporated in one ormore subbing layers or in an antihalation layer at either side of thesupport and even in the support, e.g., giving it a blue aspectpreferably offering it a specular absorption density reaching 0.45 inthe 480 to 700 nm wavelength range. The dyes have, however, preferablysuch chemical and- /or physical characteristics that they can be removedor decolourized in one of the processing baths.

According to a preferred embodiment of the present invention a filteringdye or mixture of filtering dyes absorbing in the wavelength range ofabout 480 to 600 nm is used when fluorescent screens are applied thatemit mainly green light (480-600 nm) by neutron exposure.

The amount of filtering dye is preferably in the range of 25 to 1,000 mgper sq.m but here likewise lower or higher amounts may be appropriateaccording to the result aimed at.

Suitable filtering dyes that can be removed from hydrophilic colloidlayers are e.g., those listed in table 1.

Table 'l o o n c=o HO-fi t I I H5C-CC c on CH c-- c-eH TABLE 1-Continued il n 11. -SO:H

5 7 HO- -0 S- N HOOC 0=C N H C\ H IN- -CH=&-CH3

12. "'rilterblau rumg) Farbwerke Hib'chst In the radiographiccombination of neutronfluorescent screens and silver halide radiographicmaterials used according to the present invention. the screens may bearranged separately from the lightsensitive silver halide material orthey may form with the silver halide emulsion an integral arrangement sothat on one and the same support both a silver halide emulsion and aneutron-sensitive fluorescent screen are provided. Unitary screen-silverhalide emulsion materials for X-ray recording have been described e.g.in the United States Patent Specification 2,887,379.

The emulsions may be spectrally sensitized by any of the knownprocedures. They may be spectrally sensitized by means of commonspectrally sensitizing dyes used in silver halide emulsions, whichinclude cyanine dyes and merocyanine dyes as well as other dyes asdescribed by F. M. Hamer in The Cyanine Dyes and related Compounds,lnterscience Publishers (1964). These dyes are preferably used in anamount in the range of 20 mg to 250 mg per mole of silver halide.

Suitable spectral sensitizing dyes for silver halide to be used in thecombination with screens emitting light in the wavelength range of 4S0600 nm are listed for 50 illustrative purposes in the following table 2.

Supersensitization in the green spectral range may be obtained with thefollowing compounds 22 and 23 of Table 2 in a molar ratio 1:2.

T 2 4 2 22 l H N co CH oo S -CHCH 41 3 Y Q -01; 3 N

The silver halide in the emulsion layer(s) may comprise varying amountsof silver chloride, silver iodide, silver bromide, silver chlorobromide,silver bromoiodide, and the like, but when coated must be capable, afterexposure and processing, of producing a negative silver image remainingthereon, i.e., in situ. Particularly good results are obtained withsilver bromoiodide emulsions in which the average grain size of thesilver bromoiodide crystals is in the range of about 0.1 to about 3microns.

The image-forming silver halide emulsion may be chemically sensitized byany of the known procedures. The emulsions may be digested withnaturally active gelatin or with small amounts of sulphur-containingcompounds such as allyl thiocyanate, allylthiourea, sodium thiosulphate,etc. The image-forming emulsion may be sensitized likewise by means ofreductors, e.g., tin compounds as described in the United Kingdom Pat.No. 789,823, polyamines e.g., diethyltriamine, and small amounts ofnoble metal compounds such as of gold, platinum, palladium, iridium,ruthenium, and rhodium as described by R. Koslowslty, Z.Wiss.Phot. 46,67-72 (1951). Representative examples of noble metal compounds areammonium chloropalladate, potassium chloroplatinate, potassiumchloroaurate and potassium aurithiocyanate.

Emulsion stabilizers and antifoggants may be added to the silver halideemulsion before or after admixture of the low-speed emulsion, e.g., theknown sulphinic and selenic acids or salts thereof, aliphatic, aromaticor heterocyclic mercapto compounds or disulphides, e.g.,

those described and claimed in published German Pat. No. 2,100,622,preferably comprising sulpho groups or carboxyl groups, mercurycompounds e.g., those described in Belgian Pat. No. 524,121 677,337707,386 and 709,195 and tetra-azaindenes as described by Birr inZ.Wiss.Phot. 47, 2-58 (1952), e.g., the hydroxy tetraazaindenes of thefollowing general formula:

wherein:

each of R, and R represents hydrogen, an alkyl, an

aralkyl, or an aryl group, and

R represents hydrogen, an alkyl, a carboxy, or an alkoxycarbonyl group,such as 5-methyl-7- hydroxy-s-triazolol 1,5-a]-pyrimidine.

Other additives may be present in one or more of the hydrophilic colloidlayers of the radiation-sensitive silver halide elements of the presentinvention, e.g., hardening agents such as formaldehyde, dialdehydes,hydroxy aldehydes, muccchloric and mucobromic acid, acrolein, andglyoxyal, mordanting agents for anionic colour couplers or dyes formedtherefrom, plasticizers and coating aids e.g., saponin, e.g.,dialkylsulphosuccinic acid salts such as sodiumdiisooctylsulphosuccinate, alkylaryl polyether sulphuric acids,alkylarylpolyethersulphonic acids, carboxyalkylated polyethyleneglycolethers or esters as described in French Pat. No. 1,537,417 such as isoCH, C,,1-1- ,(OCH CH,),,OCH COONa, fluorinated surfactants e.g., thosedescribed in Belgian Pat. No. 72,680 and the published German PatentApplications 1,950,121 and 1,942,665, inert particles such as silicondioxide, glass,

starch and polymethylmethacrylate particles.

For the purpose of accelerating the development, the exposedphotographic material is developed preferably in the presence ofdevelopment accelerators. These development accelerators can be usedeither in the silver halide emulsion, in adjacent layer(s) or in thedeveloping bath. They include alkylene oxide compounds of various types,e.g., alkylene oxide condensation products or polymers as described inUS. Pat. Nos.

1,970,578 2,240,472- 2,423,549 2,441,389 2,531,832 and 2,533,990 and inUnited Kingdom Pat. Nos. 920,637 940,051 945,340 991,608 and 1,015,023.Other development accelerating compounds are onium and polyoniumcompounds preferably of the ammonium, phosphonium, and sulphonium typefor example trialkyl sulphonium salts such as dimethyl-n-nonylsulphonium p-toluene sulphonate, tetraalkyl ammonium salts such asdodecyl trimethyl ammonium p-toluene sulphonate, alkyl pyridinium andalkyl quinolinium salts such as l-m-nitrobenzyl quinolinium chloride andl-dodecylpyridinium chloride, bisalkylene pyridinium salts such asN,N'-tetramethylene bispyridinium chloride, quaternary ammonium andphosphonium polyoxyalkylene salts especially polyoxyalkylenebispyridinium slats, examples of which can be found in US. Pat. No.2,944,900, etc.

After radiographic exposure the radiographic silver halide elements ofthe present invention are developed, preferably in an energetic surfacedeveloper. The high energy is required in order to allow the developmentto proceed quickly and may be obtained by properly alkalizing thedeveloping liquid (pH 9-12), by using high-energy developing substancesor a combination of developing substances, which as a consequence oftheir superadditive action is very energetic.

Economy on the silver halide in the emulsion is realized by building upthe image density partly with dyes. Such may proceed by introducing (a)colour coupler(s) into the emulsion, which at least at the stage of thedevelopment form(s) (a) dye(s) with the oxidation product of an aromaticprimary developing agent, e.g., of the p-phenylenediamine type, whichdye(s) absorb(s) in the visible part of the spectrum.

Further it is known that a relatively high maximum density and contrastcan be obtained even with a low amount of silver halide content per unitof surface when a colour image is produced together with a silver imageas is described, e.g., in the. published German Pat. No. (D.O.S.)1,946,652.

[t is further known that fine-grained silver halide emulsions have ahigher covering power than coarsegrained emulsions (ref. P. Glafkides,Photographic Chemistry, Vol. 1 (1958) 89-90). 7

By the term covering power" is understood the reciprocal of thephotographic equivalent of developed silver, i.e., the number of gramsof silver per sq. decimeter divided by the maximum optical density.

Fine-grained emulsions have a lower photographic speed. The low speed ofsaid fine-grained emulsions having a high covering power e.g., at least50, and low silver halide content, e.g., less that 80 mg of silver persq.dm, may be compensated by the use of fluorescent screens containingfluorescent gadolinium compounds and having a particularly highintensification factor.

When applying a colour development preferably socalled Z-equivalentcouplers are used to further reduce the consumption of silver thus only2 instead of4 molecules of exposed silver halide are necessary for theproduction of 1 dye molecule. Such couplers contain in the couplingposition, e.g., a halogen atom such as iodine, brominepor chlorine (seetherefor e.g., the US. Pat. No. 3,006,759). The density of the image isthus realised by addition of the densities of the silver image(s)combined with the dye image(s).

For improving the information content retrieval those phenol ora-naphthol type colour couplers are particularly suitable that on colourdevelopment of the silver halide with an aromatic primary aminodeveloping agent form a quinoneimine dye mainly absorbing in the red andalso absorbing in the green and having an absorption maximum in thespectral wavelength range of 550 to 700 (ref. therefor is made e.g., tothe published German Patent Application D.O.S. No. P 1,946,652.)

Phenol couplers suited for that purpose correspond, e.g., to thefollowing general formula:

OH l

Q-M-IR R H'llwherein:

each of R, and R represents a carboxylic acid acyl or sulphonic acidacyl group including said groups in substituted state, e.g., analiphatic carboxylic acid acyl group, an aromatic carboxylic acid acylgroup, an heterocyclic carboxylic acid acyl group, e.g., a 2-furoylgroup or a 2-thienoyl group, an aliphatic sulphonic acid acyl group, anaromatic sulphonic acid acyl group, a sulphonyl thienyl group, anaryloxy-substituted aliphatic carboxylic acid acyl group, a phenylcarbamyl aliphatic carboxylic acid acyl group, or a tolyl carboxylicacid acyl group.

For such types of phenol colour couplers and their preparation referencemay be made to US. Pat. No. 2,772,162 and 3,222,176, to United KingdomPat. No. 975,773.

When colour images are prepared together with silver images, use is madeof aromatic primary amino colour developing agents e.g.,N,N-dialkyl-p-pheny1enediamines and derivatives thereof, e.g.,N,N-diethyl-pphenylenediamine,N-butyl-N-sulphobutyl-p-phenylene-diamine, 2-amino-5-diethylaminotoluenehydrochloride, 4-amino-N-ethyl-N(B-methanesulphonamidoethyl)-m-toluidine sesquisulphate monohydrate andN-hydroxy-ethyl-N-ethyl-p-pheny1enediamine. The colour developer can beused together with black-andwhite developing agents e.g., 1-phenyl-3-pyraiolidinone and p-monomethylaminophenol which are known to have asuperadditive effect on colour development (see L. F. A. Mason,J.Phot.Sci. 11 (1963) 136-139), and other p-aminophenol derivatives,e.g. those according to French Pat. No. 1,283,420 such as3-methyl-4-hydroxy-N,N-diethylaniline, 3-methyl-4hydroxy-N-ethyl-N-l'3-hydroxyethylani1ine, 1-methy1-6- hydroxy-l,2,3,4-tetrahydroquinoline, 1 -B-hydroxyethyl--hydroxy-1,2,3,4-tetrahydroquinoline,N-(4-hydroxy-3-methylphenyl)-pyrrolidine, etc. It is also possible touse combinations of aromatic primary amino colour developing agents toobtain an increased rate of colour development (see e.g., German Pat.No. 954,311 and French Pat. No. 1,299,899); favourable effects areobtained e.g, by the use of N-ethyl-N-2- hydroxyethyl-p-phenylenediaminetogether with N- butyl-N-sulphobutyl-p-phenylenediamine, 2-amino-5-diethylamino-toluene hydrochloride or N,N-diethyl-pphenylenediaminehydrochloride.

The developing solutions may also comprise any of the usual additionalingredients e.g., sodium sulphite and hydroxylamine or derivativesthereof, hardening agents, antifoggants e.g., benzotriazole,S-nitrobenzimidazole. Smitro-indazole, halides such as potassiumbromide. silver halide solvents, toning and intensifying compounds,solvents e.g., dimethylformamide, dimethylacetamide andN-methyl-pyrrolidone for chemical ingredients that are difficult todissolve in the preparation of the developing solutions or that tend toprecipitate upon standing, etc.

The radiation-sensitive emulsions for use in the present invention maybe coated on a wide variety of supports e.g., films of cellulosenitrate, cellulose esters, polyvinylacetal, polystyrene, polyethyleneterephthalate and other polyester materials as well as a-olefincoatedpapers e.g., paper coated with polyethylene or polypropylene.

Preferred supports comprise a linear condensation polymer, polyethyleneterephthalate being an example thereof.

The supports used in the present recording materials may be coated withsubbing layers for improving the adhesion of (a) geIatino-silver halideemulsion layer(s) thereto. As already mentioned the support may becoloured. According to the present invention blue dyes are preferred.Blue polyester resin supports are known from the prior art.

The mechanical strength of melt-extruded supports of the polyester typecan be improved by stretching. In some cases as described in the UnitedKingdom Pat. No. 1,234,755 the support may carry a subbing layer in thestretching stage.

Suited subbing layers are known to those skilled in the art of silverhalide photography. With regard to the use of hydrophobic film supportsreference is made to the composition of subbing layers described in theUnited Kingdom Pat. No. 1,234,755.

According to said specification a hydrophobic film support has 1 a layerwhich is directly adherent to the said hydrophobic film support andcomprises a copolymer formed from 45 to 99.5% by weight of at least oneof the chlorine-containing monomers vinylidene chloride and vinylchloride, from 0.5 to by weight of at least an ethylenically unsaturatedhydrophilic monomer, and from 0 to 54.5% by weight of at least one othercopolymerisable ethylenically unsaturated monomer; and (2) a layercomprising in a ratio of 1:3 to 120.5 by weight a mixture of gelatin anda copolymer of 30 to 70% by weight of butadiene with at least onecopolymerisable ethylenically unsaturated monomer.

The exposed radiographic elements of the present invention arepreferably processed in an automatic processing apparatus as is used forX-ray films in which the photographic material may be guidedautomatically and at a constant speed from one processing unit to theother, but it will be understood by those skilled in the art that theradiographic image recording elements disclosed herein can also beprocessed apart from the above mentioned automatic processing apparatusin a variety of ways, such as by using the manual conventionalmulti-tank methods well known in the art.

For common emulsion preparation processes and the use of particularemulsion ingredients reference is made in general to the ProductLicensing Index of December 1971 in which the following terms are dealtwith in more details:

Emulsion type and preparation of said element Chemical sensitizationI/II III

IV Development modifiers V Antifoggants and stabililers VI Developingagents VII Hardeners VIII Binding agents or polymers for silver halidelayers and other layers IX Antistatic layers X Supports XI Plastici/ersand lubricants XII Coating aids XV Spectral sensitilation agents forsilver halides XXIII Colour material ingredients XVI Absorbing andfilter dyes XXI Physical development systems. and

XVII and XVIII Addition agents and coating procedures.

The present invention includes the use of fluorescent gadoliniumcompounds in all types of neutron detection. neutron energy conversionand neutron image recording. A very interesting use of said fluorescentcompounds lies in the conversion of neutron energy into visible light inimage-intensifier tubes and in so-called neutron-ray image converters inanalogy with X-ray image converters e.g., as described in the US. Pat.Nos. 3,403,279 and 3,617,743.

The structure of the neutron-ray image converter is basically the sameas that of the X-ray image converter.

In such neutron-ray image converter the neutron energy conversion influorescent electromagnetic radiation takes place in a vacuum or reducedpressure photo-electron producing tube containing said neutron-absorbingscreen in contact or optically coupled e.g., with fiber optics or lenssystem with a photocathode the image-wise emitted photo-electrons ofwhich are electrically and/or magnetically focused to strike aluminescent cathode-ray sensitive screen. The neutron image is thusfirst converted in a fluoresent light image which is transformed withall its variations in luminance, into an electron image" with similarvariations in density. The acceleration of the electrons in the focusingelectric field brings about essentially an intensification of the totallight" flux. In other words the greater the kinetic energy theseelectrons possess when impinging upon the cathode luminescent screen themore light they release. Therefore these tubes are also calledimage-intensifier tubes."

Suitable X-ray image conversion orintensifier tubes using gadoliniumcontaining rare earth oxysulfide phosphors and that can be used inneutron image conversion according to the present invention have been described by S. P. Wang et al. in IEEE Nuclear Science Transactions(1970). February, 49-56.

The fluorescent gadolinium compounds are suitable for the detection ofneutrons of any energy content e.g. for the detection of cold" neutrons0.01 eV), "thermal" neutrons (0.0l0.5 eV). epithermal" neutrons (0.5 eV10 keV), fast" neutrons 10 keV).

The fluorescent gadolinium compounds normally contain 15.68 of thegadolinium isotope Gd 157, which isotope is the most effective neutronabsorber known. The higher the content of that isotope in thefluorescent compound the higher the neutron absorption.

According to a preferred embodiment the fluorescent gadolinium compoundscontain gadolinium that is characterized by a higher Gd 157 content than15.68 7:. thus contain so-called isotope 157 enriched gadolinium.

The invention is illustrated in the drawing where a neutron imagerecording material comprising a support 1 covered with ananti-reflection layer 2 which contains a screening dye 6 dispersed in abinder 7 is coated with a fluorescent layer 3 containing a fluorescentsubstance 4 dispersed in a binder 5 with particular reference to thefollowing example.

Example Preparation of the light-sensitive silver halide material I usedin the comparative test as described hereinafter with the standardgadolinium metal screen.

A silver bromoiodide X-ray emulsion (2 mole of silver iodide) wasprepared in such a way that it contained silver halide grains with anaverage grain size of 0.4 p. and comprised per kg an amount of silverhalide corresponding to 190 g of silver nitrate and 74 g of gelatin.

As stabilizing agents the emulsion contained per kg 545 mg of5-methyl-7-hydroxy-s-triazolol1,5- a]pyrimidine, 6.5 mg ofl-phcnyl-S-mercaptotetrazole, and 0.45 mg of mercury cyanide.

The above emulsion was coated on one side of a subbed polyethyleneterephthalate support in such a way that on the support a silver halideemulsion layer was obtained containing an amount of silver halideequivalent to I25 g of silver nitrate per sq.m.

The emulsion layer was coated with a gelatino antistress layer at acoverage of l g/mZ.

The above defined silver halide emulsion material was made with theobject to obtain a recording material with optimum sensitivity forB-rays.

Preparation of the light-sensitive silver halide material ll used in thecomparative test as described hereinafter with the fluorescentgadolinium compound screen.

A silver bromoiodide X-ray emulsion (2 mole of silver iodide) wasprepared in such a way that it contained silver halide grains with anaverage grain size of 1.25 p. and comprised per kg an amount of silverhalide corresponding to 190 g of silver nitrate and 74 g of gelatin.

The obtained silver halide emulsion was spectrally sensitized for lightin the wavelength range of 480-600 nm with 150 mg per kg emulsion of aspectral sensitizing dye corresponding to the following structuralformula:

As stabilizing agents the emulsion contained per kg 545 mg ofS-methyl-7-hydroxy-s-triazolo[1,5- alpyrimidine, 6.5 mg ofl-phenyl-5-mercaptotetrazole, and 0.45 mg of mercury cyanide.

The above emulsion was coated on one side of 21 subbed polyethyleneterephthalate support in such a way that on the support a silver halideemulsion layer was obtained containing an amount of silver halideequivalent to 7 g of silver nitrate per sq.m.

The emulsion layer was coated with a gelatino antistress layer at acoverage of l g/m2.

Composition of the "standard screen" (metal screen for use in the directmethod").

The standard screen is a gadolinium metal foil with a thickness of 20microns.

Composition of the fluorescent screen material used according to thepresent invention in the direct method."

The fluorescent screen is composed of a fluorescent layer applied onto apolyethylene terephthalate resin support having a thickness of 250microns.

The fluorescent layer contains dispersed in a binder Gd O S activatedwith 0.3 (calculated on the gadolinium) of terbium. The fluorescentlayer is applied to an antihalation layer containing 5 mg per sq.m ofthe dye NEOZAPON FIRE RED (C.l. Solvent Red 119) in a binder adhering tothe polyester.

The fluorescent particles have an average grain size of 10 microns andare applied at a coverage of 390 grams per sq.m. The thickness of thefluorescent screen is lOl microns.

The fluorescent layer is covered with a resin type antistress layer ofIS microns.

Exposure (direct method).

A thermal neutron beam was emitted from the nuclear reactor through amonocristal of copper. Operating that way a beam of monochromaticneutrons (A 0.1065 nm) of a flux of 4. 10" neutrons per sq.cm per sec.was obtained for the exposure. The -y-ray intensity in that beam was 300milliroentgen (mr) per hour.

Both light-sensitive silver halide emulsion materials were under thesame circumstances exposed with that monochromatic" neutron beam througha test object. The test object was an iron step wedge containing threesteps (total height of the three steps l2.7 mm) having on each stepsmall objects of different size, form and neutron absorption coefficient(ref. British Journal of Applied Physics, 7 (October 1956) (page 346))viz. objects of Fe, Cd, In, Gd, polyamide, Pb and Dy.

The above described "standard screen" and the gadolinium oxysulphidescreen were during the exposure to the test object held in contactrespectively with the silver halide emulsion layer of silver halidematerial I and 1].

Test object images with same average optical density were obtained byusing for the combination of the standard screen and the silver halidematerial I a dosis of 4.2 X 10 neutrons per sq.cm and for thecombination of the fluorescent gadolinium compound screen with thesilver halide material ll only 1.7 X 10 neutrons per sq.cm so that thelast mentioned combination may be considered as being about 250 timesmore sensitive for neutron detection and recording than the combinationcontaining the standard screen."

In a further test carried out according to the transfer method" adysprosium screen was used as converter screen that becomes radioactiveand B-ray emitting after neutron bombardment. The dysprosium screen wasexposed with a neutron dosis of 3.6 X 10 neutrons per sq.cm.

The thus exposed dysprosium screen was l8 min after the neutronirradiation kept for 3 hours in direct contact with the above describedsilver halide emulsion layer of material 1. The obtained optical densitywas the same as the one obtained in the direct method" with the silverhalide material ll combined with the gadolinium oxysulphide screen whichreceived, however, a neutron dosis being 2,000 times smaller than theabove neutron dose applied onto the dysprosium screen.

We claim:

1. A method of recording neutron images which method comprises the stepsof l information-wise irradiating with neutrons an intensifying screenincorporating a phosphor compound in which gadolinium is the host metaland at least one other rare earth metal is present as a fluorescenceactivating metal; and (2) subjecting the fluorescent light patternresulting from the impact of the neutrons on the screen onto aphotographic material.

2. A method according to claim 1, wherein the gadolinium host metal isan oxide, oxyhalide or oxysulphide of gadolinium.

3. A method according to claim 1 wherein the gadolinium host metal isgadolinium oxysulphide or gadolinium oxyhalide.

4. A method according to claim 3, wherein the activating metal is atleast one of the metals of the group consisting of terbium, dysprosium,erbium, europium, holmium, neodymium, praseodymium, samarium, thuliumand ytterbium.

5. A method according to claim 1, wherein the gadolinium host metalcorresponds to the following general formula:

wherein:

M is at least one of the metals terbium, dysprosium,

erbium, europium, holmium, neodymium, praseodymium, samarium, thulium orytterbium,

X is sulphur or halogen,

n is 0.0002 to 0.2, and

w is I when X is halogen or is 2 when X is sulphur.

6. A method according to claim 1, wherein the gadolinium host metal isgadolinium oxysulphide activated with terbium.

7. A method according to claim 1, wherein the gado linium host metalcontains gadolinium with a Gd 157 isotope content larger than 15.68

8. A method according to claim 1, wherein the intensifying screen is inthe form of a layer applied to a support or is a self-supporting layeror sheet.

9. A method according to claim 8, wherein the layer or sheet containsthe gadolinium host metal in the form of particles dispersed in abinder.

10. A method according to claim 9, wherein said particles have a size inthe range of l to 20 microns.

11. A method according to claim 9, wherein the layer has a thickness ofto 300 microns.

12. A method according to claim 9, wherein the binder is present in saidlayer in a proportion of 5-15 by weight with respect to the fluorescentmaterial.

13. A method according to claim 1, wherein the intensifying screenincluding said phosphor compound is used in conjunction with anotherneutron-absorbing screen of the group of metallic, glassy or granulartype screens.

14. A method according to claim 1, wherein the intensifying screen isused in conjunction with a metal screen that becomes radioactive byneutron bombardment.

15. A method according to claim 1, wherein the intensifying screen isused in conjunction with a metal screen having a neutron absorptioncoefficient at least as high as lithium.

16. A method according to claim 1, wherein the photographic materialincludes a photosensitive silver halide emulsion layer coated on asupport.

17. A method according to claim 1, wherein the photographic materialincludes a silver halide emulsion layer on each side of a support.

18. A method according to claim 16, wherein said photographic materialcontains a filtering dye or dyes improving the sharpness of the silverimage obtainable in said material.

19. A method according to claim 18, wherein said dye or dyes have suchchemical and physical characteristics that they can be removed ordecolourized in one of the processing baths used for the photosensitivesilver halide material.

20. A method according to claim 18, wherein said dye or dyes areincorporated in a hydrophilic colloid layer.

21. A method according to claim 20, wherein the dye or dyes areincorporated in a hydrophilic colloid layer between the silver hadideemulsion layers when using a material with duplitized coating or in thesilver halide emulsion layers themselves.

22. A method according to claim 18, wherein the dye or dyes areincorporated in one or more subbing layers or in an antihalation layerat either side of the support.

23. A method according to claim 18, wherein the dye or dyes are presentin the support.

24. A method according to claim 16, wherein the intensifying screen andthe photo-sensitive silver halide material form an integral arrangement.

25. The method of claim 16, wherein the intensifying screen is arrangedseparately from the photo-sensitive material containing the silverhalide.

26. A method according to claim 16, wherein the silver halide has beenspectrally sensitized.

27. A method according to claim 16, wherein the silver halide is asilver bromoiodide having an average grain size in the range of about0.1 to 5 u.

28. A method according to claim 16, wherein the silver halide emulsionlayer(s) contain(s) a colour coupler for forming a dye with an oxidizedpphenylenediamine developing agent.

29. A method according to claim 16, wherein the photosensitive silverhalide material contains an amount of silver halide equivalent to above3 to 8 g silver per sq.m.

1. A METHOD OF RECORDING NEUTRON IMAGES WHICH METHOD COMPRISES THE STEPSOF (1) INFORMATION-WISE IRRADIATING WITH NEUTRONS AN INTENSIFYING SCREENINCORPORATING A PHOSPHOR COMPOUND IN WHICH GADOLINIUM IS THE HOST METALAND AT LEAST ONE OTHER RARE EARTH METAL IS PRESENT AS A FLUORESCENCEACTIVATING METAL; AND (2) SUBJECTING THE FLUORESCENT LIGHT PATTERNRESULTING FROM THE IMPACT OF THE NEUTRONS ON THE SCREEN ONTO APHOTOGRAPHIC MATERIAL.
 2. A method according to claim 1, wherein thegadolinium host metal is an oxide, oxyhalide or oxysulphide ofgadolinium.
 3. A method according to claim 1 wherein the gadolinium hostmetal is gadolinium oxysulphide or gadolinium oxyhalide.
 4. A methodaccording to claim 3, wherein the activating metal is at least one ofthe metals of the group consisting of terbium, dysprosium, erbium,europium, holmium, neodymium, praseodymium, samarium, thulium andytterbium.
 5. A method according to claim 1, wherein the gadolinium hostmetal corresponds to the following general formula: Gd(w n).MnOwXwherein: M is at least one of the metals terbium, dysprosium, erbium,europium, holmium, neodymium, praseodymium, samarium, thulium orytterbium, X is sulphur or halogen, n is 0.0002 to 0.2, and w is 1 whenX is halogen or is 2 when X is sulphur.
 6. A method according to claim1, wherein the gadolinium host metal is gadolinium oxysulphide activatedwith terbium.
 7. A method according to claim 1, wherein the gadoliniumhost metal contains gadolinium with a Gd 157 isotope content larger than15.68 %.
 8. A method according to claim 1, wherein the intensifyingscreen is in the form of a layer applied to a support or is aself-supporting layer or sheet.
 9. A method according to claim 8,wherein the layer or sheet contains the gadolinium host metal in theform of particles dispersed in a binder.
 10. A method according to claim9, wherein said particles have a size in the range of 1 to 20 microns.11. A method according to claim 9, wherein the layer has a thickness of10 to 300 microns.
 12. A method according to claim 9, wherein the binderis present in said layer in a proportion of 5-15 % by weight withrespect to the fluorescent material.
 13. A method according to claim 1,wherein the intensifying screen including said phosphor compound is usedin conjunction with another neutron-absorbing screen of the group ofmetallic, glassy or granular type screens.
 14. A method according toclaim 1, wherein the intensifying screen is used in conjunction with ametal screen that becomes radioactive by neutron bombardment.
 15. Amethod according to claim 1, wherein the intensifying screen is used inconjunction with a metal screen having a neutron absorption coefficientat least as high as lithium.
 16. A method according to claim 1, whereinthe photographic material includes a photosensitive silver halideemulsion layer coated on a support.
 17. A method according to claim 1,wherein the photographic material includes a silver halide emulsionlayer on each side of a support.
 18. A method according to claim 16,wherein said photographic material contains a filtering dye or dyesimproving the sharpness of the silver image obtainable in said material.19. A method according to claim 18, wherein said dye or dyes have suchchemical and physical characteristics that they can be removed ordecolourized in one of the processing baths used for the photosensitivesilver halide material.
 20. A method according to claim 18, wherein saiddye or dyes are incorporated in a hydrophilic colloid layer.
 21. Amethod according to claim 20, wherein the dye or dyes are incorporatedin a hydrophilic colloid layer between the silver hadide emulsion layerswhen using a material with duplitized coating or in the silver halideemulsion layers themselves.
 22. A method according to claim 18, whereinthe dye or dyes are incorporated in one or more subbing layers or in anantihalation layer at either side of the support.
 23. A method accordingto claim 18, wherein the dye or dyes are present in the support.
 24. Amethod according to claim 16, wherein the intensifying screen and thephoto-sensitive silver halide material form an integral arrangement. 25.The method of claim 16, wherein the intensifying screen is arrangedseparately from the photo-sensitive material containing the silverhalide.
 26. A method according to claim 16, wherein the silver halidehas been spectrally sensitized.
 27. A method according to claim 16,wherein the silver halide is a silver bromoiodide having an averagegrain size in the range of about 0.1 to 5 Mu .
 28. A method according toclaim 16, wherein the silver halide emulsion layer(s) contain(s) acolour coupler for forming a dye wIth an oxidized p-phenylenediaminedeveloping agent.
 29. A method according to claim 16, wherein thephotosensitive silver halide material contains an amount of silverhalide equivalent to above 3 to 8 g silver per sq.m.